Many techniques to discriminate the effects between strain and temperature in fiber Bragg grating (FBG) sensors have been reported, which include schemes using a tilted fiber Bragg grating demodulator [Sung Chul Kang, Se Yoon Kim etl, “Temperature-independent strain sensor system using a tilted fiber Bragg grating demodulator,” IEEE Photon. Technol. Lett., vol. 10, No 10, October 1998], superimposed gratings [M. G. Xu, J. L Archambault, L. Reekie, and J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett., vol. 30, no. 13, pp. 1085-1087, 1994], a chirped Bragg grating [M. G. Xu, L. Dong, L. Reekie, J. A. Tucknott, and J. L. Cruz, “Temperature-independent strain sensor using a chirped Bragg grating in a tapered optical fiber,” Electron. Lett., vol. 31, no. 10, pp. 823-825, 1995] and a long period grating [V. Bhatia, D. Campbell, R. O. Claus, and A. M. Vengsarkar, “Simultaneous strain and temperature measurement with long-period gratings,” Opt. Lett., vol. 22, no. 9, pp. 648-650, 1997], etc.
Among existing schemes, a dual head sensor is one of the most effective schemes due to its compact size and good performance [S. W. James, M. L. Dockney, and R. P. Tatam, “Simultaneous independent temperature and strain measurement using in-fiber Bragg grating sensors,” Electron. Lett., vol. 32, no. 12, pp. 1133-1134, 1996] [M. Song, S. B. Lee, S. S. Choi, and B. Lee, “Simultaneous measurement of temperature and strain using two fiber Bragg gratings embedded in a glass tube,” Opt. Fiber Technol., vol. 3, no. 2, pp. 194-196, 1997]. However, those known techniques are more research-based, and none of those techniques provide proper packaging methods that can cater for real applications. Although the techniques using an FBG [D. A. Jackson, A. B. Lobo Ribeiro, L. Reekie, and J. L. Archambault, “Simple multiplexing scheme for a fiber-optic grating sensor network,” Opt. Lett., vol. 18, no. 14, pp. 1192-1194, 1993] or a fiber Fabry-Perot wavelength filter [A. D. Kersey, T. A. Berkoff, and W. W. Morey, “Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter,” Opt. Lett., vol. 18, no. 16, pp. 1370-1372, 1993] as a demodulator have shown good results, they need a complete temperature-isolation of the demodulating FBG or a sophisticated calibration in fabricating the filter, respectively.
Various methods have been devised for achieving temperature independence for the wavelengths of FBGs. These methods include active systems that utilize feedback to monitor and dynamically control certain parameters, and passive devices that utilize the thermal characteristics of materials/structures to modify the response of the FBG wavelength to temperature. Passive devices are more desirable since they are much simpler and no power source and so are generally maintenance-free. The wavelength of an FBG is determined by the refractive index of the fiber and the period of the grating, both of which change with temperature. Since the refractive index is hard to control, passive temperature compensation devices generally operate by controlling the elongation with temperature of the optical fiber containing the FBG.
The control is usually accomplished by embedding the fiber containing the FBG into a mechanical structure, which is designed to release tension applied to the fiber with increasing temperature. G. W. Yoffe et al [G. W. Yoffe, Peter A. Krug, F. Ouellette, and D. A. Thomcraft, “Passive temperature-compensating package for optical fiber grating,” Applied Optics, Vol. 34, No. 30, 20 Oct. 1995] has proposed a passive temperature compensating package for an optical fiber grating, in which the grating is mounted under tension in a package comprising two materials (a silica tube and an aluminum tube) with different thermal-expansion coefficients (TEC). As the temperature rises, the strain is progressively released, compensating the temperature dependence of the Bragg wavelength. A wavelength shift of 0.7 pm/° C. was achieved, but the overall structure requires precision-made components and is complicated to assemble. Another complicated design based on a similar principle to offer temperature compensation over a wider temperature range has been patented by Lin et al., “Temperature-compensating device with tunable mechanism for optical fiber gratings,” (U.S. Pat. No. 6,374,015, 2002).
In contrast to the electrical strain sensors, optical fiber sensors have many advantages from the viewpoint of the principle such as small size and robustness, immunity to EMI and RFI (Radio Frequency Interference), inherent safety (explosion-proof) and accuracy. However, there remain the above mentioned problems to be overcome in the development of optical fiber sensors.
A need therefore exists to provide an optical fiber sensor that seeks to address at least one of the above mentioned problems.